1. Field of-the Invention
This invention relates to a method and apparatus for multiplex transmission in which data frames are transmitted by means of a transmission technique called CSMA/CD (carrier sense multiple access/collision detection).
2. Description of the Related Art
In the aforementioned type of multiplex transmission methods, a plurality of multiplex transmission apparatuses is connected to a transmission line (bus) comprising, for example, a paired cable, and each multiplex transmission apparatus is composed at least of a communication control circuit (LSI), a receiving buffer memory (hereinafter referred to as "receiving buffer"), a port control circuit, and an output port. The LSI writes data of a frame format, supplied from the bus, in the receiving buffer, and then the port control circuit selects an output port in accordance with the contents of the written data, so as to output the data to a specific load to be controlled (hereinafter merely referred to as "load").
The multiplex transmission apparatuses interconnected by the bus constitute a network and employs, as the transmission technique, CSMA/CD (carrier sense multiple access/collision detection). The network composed of these multiplex transmission apparatuses is used, for example, as a local area network (LAN) for carrying out multiplex transmission of data within a motor vehicle.
Each multiplex transmission apparatus is unable to detect an erroneous state of its component parts, such as stoppage of the oscillation of a clock pulse generator for generating clock pulses in accordance with which the communication control circuit operates, or a fault of the communication control circuit itself. In such cases, unexpected data may appear at the output port, possibly causing abnormality of the load connected to the output port.